1. Field of the Invention
The present invention relates to a color filter used, for example, in a color liquid crystal display device, and a liquid crystal display device provided with the same.
2. Description of the Related Art
In recent years, a color liquid crystal display device has been used in various applications such as a monitor for personal computers (PCs), a display for cell phones, laptop PCs and personal digital assistants, because of advantages such as lower power consumption and space saving, and has also been used in liquid crystal televisions in recent years as a substitute for a conventional CRT-based television.
In liquid crystal television applications, color reproducibility is considered important. The color reproducibility of a color liquid crystal display device is determined by the colors of lights emitted from red, green and blue filter segments and is evaluated on the basis of the area of a triangle (hereinafter referred to as area A) defined by the three chromaticity points of red, green and blue filter segments expressed as (xR, yR), (xG, yG) and (xB, yB) on the x-y chromaticity diagram. More specifically, the color reproducibility is expressed as the ratio of the area A to the area (standard area) of the triangle defined by 3 points of red (0.67, 0.33), green (0.21, 0.71) and blue (0.14, 0.08) that are the three primary colors in the standard system prescribed by U.S. National Television System Committee (NTSC) ((area A/standard area)×100; the unit is %; hereinafter abbreviated as NTSC ratio). The NTSC ratio is about 40 to 50% for general laptop computers, about 50 to 60% for PC monitors and about 72% for liquid crystal televisions.
In order to increase the NTSC ratio, it is necessary to increase the color purity of each filter segment. However, as the color purity of the filter segment is increased, the light utilization efficiency of a backlight (represented by the lightness Y value) is decreased. Therefore, high electric power is required in order to increase the Y value while maintaining high color purity. Among conventional color liquid crystal display devices, products powered mainly with a battery, such as personal digital assistants and cell phones, do not use high electric power because of an emphasis on power consumption, and thus the NTSC ratio thereof is as low as 30 to 50%. However, as the opportunity of watching photos and TV programs on personal digital assistants and on displays of cell phones is increasing in recent years, there is increasing demand for a higher NTSC ratio even in personal digital assistants and displays of cell phones. A cold-cathode tube backlight is used as a backlight in laptop PCs, monitors and liquid crystal televisions, while a pseudo-white LED of lower power consumption formed by coating the surface of a blue LED with a phosphor is often used as a backlight in displays of products powered mainly with a battery such as personal digital assistants and cell phones.
The pseudo-white LED formed by coating the surface of a blue LED emits white light as a result of the blue light emitted by the blue LED passing through the phosphor-coated layer. The phosphor-coated layer has a function of converting the blue light to a light of a longer wavelength. All the blue lights emitted by the blue LED are not converted to the light of the longer wavelength. Thus, the pseudo-white LED emits a mixed light of the remainder blue light and the light of the longer wavelength, which is recognized as white light by an observer.
Therefore, the pseudo-white LED formed by coating the surface of a blue LED is different in wavelength and spectrum peak from the cold-cathode tube backlight as the light source. The spectral characteristics of the three color filters of red, green and blue used in conventional color liquid crystal display devices have been designed so as to be adapted to the light-emitting characteristics of the cold-cathode tube backlight.
In order to improve the color reproducibility when the pseudo-white LED is used, filter segments are required, which have spectral characteristics different is from those of the filter segments used in the cold-cathode tube backlight. Techniques of improving color reproducibility where white LED is used are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-177592 for a green filter segment and in Jpn. Pat. Appln. KOKAI Publication No. 2004-145275 for a red filter segment.
However, not only the color reproducibility of red, green and blue colors, but also the color reproducibility and lightness (Y value) of white color are important in the color liquid crystal device. The color reproducibility and lightness (Y value) of white color cannot be improved by merely improving the color reproducibility of each filter segment.